Sludge disposal for many cities in the United States has become a problem of increasing dimensions in the last two decades. With the highest per capita sludge generation in the world, the U.S. now generates about 4.5 billion dry kg per year in sewage solids which must be disposed of or utilized in a manner posing the least amount of threat to the environment. At present, most of this sludge is disposed of by means of landfill, ocean dumping and incineration; relatively small amounts are applied to agricultural lands. In light of emerging environmental, energy and economic constraints, land application now appears to be one of the most viable sludge utilization and disposal alternatives for the future. However, present sludge management technology often cannot guarantee public health protection or cost effective solutions.
For years, the extraction of heavy metals from sewage sludges and sludge/soil mixtures with acid washings has been used as a standard laboratory test to determine the extent of metal uptake likely to occur with plants grown on sludge-amended soils. Laboratory tests have been conducted on untreated and anaerobically treated sludge using acid extraction techniques to remove heavy metals. These studies have shown that the direct acidification of primary, waste-activated, or anaerobically digested sludge cannot consistently achieve a rapid and quantitative solubilization of heavy metals. While the data reported in the prior art seem to indicate that a processing scheme consisting of acid treatment combined with dewatering may be capable of some degree of removal of heavy metals from municipal sewage sludge, previous systems do not disclose the efficient, low-cost method of heavy metals solubilization by oxidation-reduction potential adjustment and controlled-environment acidification.
The removal of heavy metals from municipal sewage sludge, however, has seldom been emphasized as a necessary treatment step. The presence of heavy metals, particularly cadmium, at significant levels in sludges of municipal treatment plants receiving industrial discharges has become a topic of increasing concern in the light of the potential impact of heavy metals on the environment and the food chain. Within the decade, evidence has been accumulated indicating that cadmium in sludge-amended soils can be magnified and accumulated in the food chain. Of further concern, cadmium has been reported to accumulate in the human kidneys to life-time levels not far below concentrations that would be expected to produce damage to this organ. Other toxic heavy metals known to be present in municipal sewage sludge include chromium, copper, lead, nickel and zinc. It is well established that heavy metals can be removed from dilute wastewaters passing through a municipal treatment plant with a high degree of efficiency. In typical municipal treatment plants accepting effluent discharges from industry, heavy metals in the treated sewage can be either adsorbed to organic particles and settled out in primary treatment or entrapped in the biological floc of the activated sludge process. Heavy metals can thus be removed from the sewage stream in primary and secondary treatment with combined efficiencies ranging from 60 to 90 percent in most cases. This results in a sludge with heavy metal concentrations 10 to 100 times greater than concentrations measured in the influent sewage. The metals in the resultant sludge are usually predominant in their insoluble form as precipitates of hydroxide, carbonate, phosphate or sulfide, especially when heavy metal concentrations exceed 10.sup.-4 M and when pH values at or above neutraliy are maintained. Sludges subject to prolonged thickening, storage, or anaerobic digestion, however, would have oxidation reduction potentials (ORP) in the range considered anaerobic, below -330 mv. Inorganic chemistry theory would predict that at low ORP and at near neutral pH, heavy metals such as cadmium, copper, nickel, iron, lead and zinc exist at equilibrium primarily as insoluble sulfide pricipitates.
The conversion of sludge heavy metals to the soluble form prior to physical separation would allow the removal of heavy metals with conventional dewatering techniques in use today (i.e., centrifugation, belt press, dewatering beds, vacuum filter, etc.).
The present invention represents an improvement over the combined biological-chemical detoxification methods of the Hayes et al U.S. Pat. No. 4,277,342 in the name of the same assignee as the present invention. In this prior Hayes et al patent, it was disclosed that the solubilization of heavy metals in a mixture of thickened primary sludge and secondary waste activated sludge could be achieved by first raising the oxidation reduction potential (ORP) of the sludge mixture followed by controlled acidification to pH 1.0-3.0 while maintaining the ORP above 0 mv. It was also reported that a biological reactor, called the "aerobic autoheated thermophilic digestion" (AATD) system, could be used to achieve both pasteurization of the sludge and the elevation of ORP required for rapid heavy metals solubilization. Results from bench scale experiments presented in that disclosure indicated that AATD treatment combined with "controlled environment acidification" to pH 1.0-3.0 could achieve nearly quantitative solubilization of heavy metals such as cadmium, zinc, and nickel, within 6.0 hours of acidification.
Although the initial performance data appeared promising, it was recognized that the biological processing stage, representing more than 85 percent of the total tank volume of the system, would comprise a substantial portion of the capital cost of a sludge metals extraction facility. It was also apparent that the biological/chemical detoxification invention was most applicable to integration into new or expanding sewage treatment facilities where the AATD system would not represent a redundant add-on process to existing municipal sludge stabilization systems, most of which employ anaerobic digestion. In addition, oxygen residuals that were necessary to maintain biological growth in aerobically digesting sludge were far higher than oxygen levels required to achieve ORPs favorable for metals solubilization. These considerations stressed the need for an improvement of the first invention which would eliminate the biological treatment system and thereby reduce tank volume and energy requirements for metals extraction.